The 8606 should show the earnings as taxable. Review the 8606 lines 1-15 for an error in your entries. Or the "Taxable IRA Distribution Worksheet" if there is an * next to line 15.
Your total basis shoukd be $12,000 that offsets $12,000 leaving the gains as taxable on line 18.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**